I just ordered the flat pack for the Scavenger drives from Big Blue Saw. This seems an easy approach to getting strong, fast feet: the drives use the same E3-150 motors as the more expensive Warp Drives, and omni-wheels can also be used to improve turning and spinning. A set of warp drives costs $1080 currently, but all in with parts, the Scavengers should come in around $450, and that difference buys a lot of other parts.
Ian (my older son) and I are about to launch into careful documentation of the Com8 frame to reproduce it in Fusion 360. From there, we'll design our own center foot lifting mechanism. I've opted not to use the Com8 mechanism because this is something I really want us to design and build ourselves. I doubt we will save any money doing our own, but we'll learn a lot, and if there's a purpose to this project, that's it.